Physical mapping of rice chromosomes 8 and 9 with YAC clones

First efforts for physical mapping of rice chromosomes 8 and 9 were carried out by ordering YAC clones of a rice genomic DNA library covering six genome equivalents with mapped DNA markers. A total of 79 and 74 markers from chromosomes 8 and 9, respectively, were analyzed by YAC colony and Southern hybridization using RFLP markers of cDNA and genomic clones, and by polymerase chain reaction (PCR) screening using PCR-derived and sequence-tagged site (STS) markers. As a result, 252 YAC clones were confirmed to contain the mapped DNA fragments on both chromosomes. A contig map was constructed by ordering these YAC clones and about 53% and 43% genome coverage was obtained for chromosomes 8 and 9, respectively, assuming a YAC clone size of 350 kb and overlap between neighboring YACs of 50%. A continuous array of YAC clones with minimum overlap gave a total size of 18.9 Mb for chromosome 8 and 15.6 Mb for chromosome 9, which are close to previous estimates. These contig maps may provide valuable information that can be useful in understanding chromosome structure and isolating specific genes by map-based cloning.